Physical vapor deposition (PVD) techniques are widely used to deposit high quality coatings for a wide range of industries, ranging from microelectronics and liquid crystal displays (LCDs) to architectural glass and DVDs. The need for high performance coatings has increased steadily over the years. Sputtering, which involves the transfer of a material from a sputtering target to a substrate, is one of several methods used for the PVD of thin films.
To initiate sputtering, the sputtering target is biased with a negative voltage. A sputtering source (or “gun”) supplies the electrical means to bias the target. A gas such as argon (the “working gas”) is ionized between the cathode (target) and the anode, forming a plasma containing positive argon ions and free electrons. The plasma is characterized by a visible glow and increased electric conductivity.
The free electrons in the plasma may be accelerated toward the anode, colliding with other gas atoms in their path and further ionizing the gas. The negatively charged target attracts the positive ions from the plasma. The positive ions are accelerated to a high kinetic energy and strike the surface of the target structure, where part of the kinetic energy is converted to heat, and the remainder of the energy is imparted to atoms of the target material by momentum transfer. Target atoms that gain sufficient energy to overcome their binding energy escape from the target surface and may be deposited on a substrate or object placed in opposition to the target, forming a coating.
In addition to biasing the target, the sputtering source may allow cooling of the target structure and may provide a magnetic field to contain and enhance the plasma. The magnetic field causes electrons in the plasma to follow curved trajectories. A longer (curved) path increases the probability of collision with gas atoms in the chamber. Such collisions produce additional gas ions, thereby increasing the sputtering rate. The deposition rates achieved by magnetron sputtering are typically several orders of magnitude higher than those attained by conventional sputtering processes, and the magnetic field configuration above the target may allow a stable discharge current to be maintained at lower pressures.
Ionized PVD (iPVD) is a class of PVD where the deposition flux from the sputtering target includes an increased fraction of ionized material. Since ion energy and direction can be controlled, iPVD techniques may be used in a wide variety of applications such as the formation of diffusion barriers and seed layers on the side and bottom of high aspect ratio trenches and vias in the microelectronics industry.
High power pulsed magnetron sputtering (HPPMS) is a type of iPVD technique where short, high peak power pulses are applied to the sputtering target at low duty cycles, which may generate plasma electron densities as high as 1019 m−3 above the target surface during the power pulses. Such high electron densities near the target may enhance ionization of sputtered material. Some of the ionized sputtered material may be accelerated back to the target, resulting in an increase in the sputtering rate, while some sputtered ions may escape and be deposited on the substrate as a high quality film. Thin films deposited using the HPPMS technique may be denser and smoother and may have better adhesion to the substrate than films deposited by direct current (DC) magnetron sputtering.
Conventional magnet assemblies for magnetron sputtering have one of two magnetic field configurations: an unbalanced or a balanced magnetic flux, as illustrated in FIGS. 1A and 1B, respectively. These configurations can be distinguished by the coefficient of unbalance K, which is the ratio of magnetic fluxes from the central and peripheral magnets on the target surface:
  K  =                    Φ        1                    Φ        2              =                            ∫                      S            1                          ⁢                              B                          ⊥              1                                ⁢          d          ⁢                                          ⁢          S                                      ∫                      S            2                          ⁢                              B                          ⊥              2                                ⁢          d          ⁢                                          ⁢          S                    
where B⊥1 and S1 are the component of the magnetic field perpendicular to the target surface and the cross-sectional area of the outer magnets, respectively, and where B⊥2 and S2 are the component of magnetic field perpendicular to the target surface and the cross-sectional area of the central magnets, respectively. When the magnetic fluxes of outer and central poles are equal (K=1), all lines of the magnetic field are closed between the poles. For K>1, the magnetic flux at the outer pole exceeds the magnetic flux at the central pole, and some of the magnetic lines at the outer pole are open, producing the unbalanced configuration.
The coefficient K reflects the magnetic field line topology, which influences the deposition rate and ion/atom flux distribution on the substrate. These conventional configurations have at least one element in common: the magnetic field above the target has a non-zero component parallel to the target across the entire target. The resulting magnetic field, in conjunction with the electric field, acts as a trap to direct ions back to the target.
The DC magnetron sputtering process is well studied and there are mathematical models to predict and optimize the deposition rates in these discharges. Significantly lower deposition rates have been reported for HPPMS compared to DC magnetron sputtering. Some of the possible reasons for this are: (1) the “return effect” of metal ions, where the sputtered metal ions are attracted back towards the target; (2) magnetic confinement influences on deposition rate and (3) movement of sputtered ions in a sideways direction from the target. Once the HPPMS discharge gets to a state with higher fraction of ionized sputtered material, the electric field distribution inside the plasma may change and prevent ions from escaping the plasma region. The plasma electric potential during the discharge may control the movement of ions, and thus it may be important to modify the electric potential distribution in the plasma during discharge to increase the ion flow from the trap.